1. Field of the Invention
The invention relates in general to copolymers for which the energy levels (HOMO and LUMO) and color of emission can be adjusted or tuned by choosing a proper composition, and electronic devices in which the active layer includes such polymeric materials.
2. Description of the Related Art
The use of organic light-emitting displays is increasing as a replacement to liquid crystal displays (LCD) and thin-film transistor (TFT) displays in many applications. Patents and articles address ways to maximize the quantum efficiency of the OLED as well as to change the color of the light emitted from organic light-emitting diodes (OLEDs).
Balanced carrier injection from both the cathode and anode can increase the quantum efficiency of the LED device. Carrier injection can be sensitive to the height of the barriers in the interface between an electrode and the polymer in an organic LED. The energy levels of a conjugated polymer, which are the highest occupied molecular orbital (“HOMO”) and lowest unoccupied molecular orbital (“LUMO”), may be tuned so that a substantially balanced and nearly maximum amount of the carriers can be injected from the electrodes.
In some references, the color emitted by the display can be tuned by changing the band gap (energy difference between HOMO and LUMO of the light emitting polymer). The change can be accomplished by using different side chains (e.g., change from alkyl to alkoxy) or by using a different monomeric unit. An example of the latter may include a polymer with a single repeating structure represented by (A-B)n, where “A” and “B” are monomeric units of the polymer, and n is the number of repeating units. As one example, the “A” unit may include one, two, or three thiophene rings in different polymer compositions. When “A” has one thiophene ring, the polymer may emit blue light. When “A” has two and three thiophene rings, the polymer may emit green light and yellow light, respectively. Unfortunately, thiophene tends to emit light at a lower intensity compared to other light-emitting polymers. Therefore, even if it gives the desired color, the intensity of light at that particular range of wavelengths may be too low for use in an OLED.
In still other attempts to affect the light emitted from an organic LED, a low molecular weight dye may be doped into a polymer host. From a practical standpoint, however, low molecular weight dyes tend to crystallize, causing phase separation which results in poor electronic devices.